In a field of electronics mounting technology, semiconductors, functional parts and/or the like are arranged on a circuit substrate and connected making the best use of film-forming technology, micro-connection technology, sealing technology and the like, and these are three-dimensionally incorporated together with other component parts into a basket-like package to fabricate an electronic apparatus having necessary performance. In packages of semiconductor chips such as IC and LSI, various mounting system are developed.
There has heretofore been developed a wire bonding method that electrodes formed in a semiconductor chip are electrically connected to inner leads of an external wiring with bonding wires within a semiconductor package, and the package is sealed with an epoxy resin, ceramic or the like. In recent years, the degree of integration of semiconductor chips has improved year by year, and the number of terminals has increased attending on this. Therefore, packages of the area array lead type such as BGA (ball grid array) applying the technique of a multi-layer printed wiring board have been developed have been developed for the purpose of miniaturizing the package to achieve high-density mounting.
Further, there has recently been a strong demand for rationalization of wiring, such as rationalization within a device, space saving and improvement in assembling ability, in addition to miniaturization, weight saving and improved performance of electronic apparatus. MID with wirings three-dimensionally formed on a surface of an injection-molded article has been developed against such a demand for rationalization of wiring. In MID, unlike the printed wiring board, a plating film (plating layer) is used in place of a copper foil to form a circuit.
In order to produce MID, injection molding is conducted with a resin of plating grade, the whole surface of the resultant article is roughened, and a catalyst is then coated to form a plating film. The formation of the plating film is generally conducted by electroless plating or electroless plating and electroplating subjected thereon. A pattern is then formed on the plating film by photolithography using a resist to form a circuit. Examples of a method for forming the circuit include a subtractive method using an etching resist and a semi-additive method using a plating resist. It may also be possible that after a plating film is formed on the whole surface of the article, unnecessary portions of the plating film are removed by laser abrasion to form the circuit.
As another production process of MID, a platable resin is molded to produce an article for three-dimensional circuit, and a catalyst is applied to the surface of the article. A hardly platable resin is arranged on a portion of the article, on which plating is unnecessary, by molding to produce an integral article. Lastly, the catalyst-coated portion is plated by a full-additive method to form a circuit. The hardly platable resin integrally molded plays a role of a plating resist upon the plating.
As described above, MID is such that a circuit (conductive pattern) is formed on the surface of an article molded from a thermoplastic resin or thermosetting resin utilizing a wet plating process such as electroless plating or electroplating. MID is a wiring board having a function as a structure member or mechanical part and a function as a wiring member in combination. Since a resin article is produced by a melt molding process such as an injection molding process, the form of the article can be freely designed, and the article is also excellent in productivity. In addition, since the plating layer can be formed on any surface of the article, the circuit can be three-dimensionally formed, and so it is convenient for MID to use as a package for a semiconductor chip.
MID can be not only utilized as a package for a semiconductor chip, but also integrally combined with peripheral circuit parts and mechanical parts into one part, so that it has a merit that compact and rational mounting design becomes feasible. Therefore, MID is intended to develop its uses in a wide variety of fields such as package cases for electronic device parts, wire-rationalized products and adaptors for hybrid IC.
Even in MID, as with the printed wiring board, surface-mounting parts such as semiconductor chip parts and flat package ICs are soldered by reflow soldering. In the surface mounting by the reflow soldering, after printing is conducted in advance on a land (pad) with solder paste, electrodes of semiconductor chip parts, or the like are mounted thereon, and finally the solder paste is melted by means of a reflow oven to conduct the soldering.
Accordingly, MID is required to have heat resistance (namely, reflow resistance) that can withstand the mounting by reflow soldering. More specifically, MID is required to have high heat resistance to the extent that it is not deformed at a temperature of at least 240° C., preferably about 260° C. Therefore, as a resin material making up MID, is used a super engineering plastic such as a liquid crystal polymer (LCP).
As a process for forming a plating layer on the surface of an LCP article, is adopted, for example, (1) a process that after a resin material with an inorganic filler dispersed in LCP is injection-molded, and the resultant article is etched with an aqueous solution of sodium hydroxide to form fine irregularities in the surface thereof, a palladium catalyst is carried on the irregularities to grow a plating layer formed of copper, nickel, gold or the like thickly by only electroless plating, or (2) a process that after a thin plating layer is formed on the surface of the article by electroless plating, the plating layer is grown thickly by electroplating. The thickness of the plating layer is generally designed to about 5 to 30 μm from the viewpoint of conductor resistance. The fine irregularities formed in the LCP article become anchor holes for the plating layer and play a role of ensuring the adhesion strength of the plating layer to the surface of the article.
The adhesion strength of the plating layer is measured in accordance with, for example, a method illustrating in FIG. 1. A metal wire 4 having a diameter φ of 1.5 mm is vertically stood on a plating layer 2 formed on the surface of an article 1, and one end of the metal wire 4 is soldered. The diameter φ and height of a soldered joint 3 are set to 4 mm and 2 mm, respectively. The other end of the metal wire 4 is pulled at a crosshead speed of 10 mm/min to peel the plating layer 2 in a direction perpendicular to the surface of the plating layer 2 through the soldered joint 3, thereby measuring peel strength at this time. The adhesion strength between the article and the plating layer is required to indicate at least 2 MPa by the above-described test method from the viewpoint of reliability.
In the application to the above-described semiconductor package or the like, of which wire bonding is required, the surface roughness of the plating layer is required to have high smoothness of at most 1 μm in terms of arithmetic mean roughness Ra. If the smoothness of the plating layer surface is poor, in some cases, the bonding strength of a bonding wire may be lowered, and the bonding may become unstable or further infeasible.
In the conventional LCP article, however, it has been extremely difficult to satisfy the adhesion strength between the article and the plating layer and the smoothness of the plating layer surface at the same time. When the etching time of the LCP article is shortened to make the fine irregularities formed therein small, the smoothness of the plating layer surface can be improved to lower the arithmetic mean roughness Ra to 1 μm or lower. However, its adhesion strength is greatly lowered to fail to ensure the adhesion strength of at least 2 MPa. On the other hand, when the etching time of the LCP article is lengthened to make the fine irregularities formed therein large, the adhesion strength of the plating layer can be enhanced, but the smoothness of the plating layer is impaired.
From the reasons described above, there has not yet been realized any article having smoothness of at most 1 μm in terms of arithmetic mean roughness Ra and adhesion strength of at least 2 MPa to the article in combination and additionally having reflow resistance by which mounting by reflow soldering becomes feasible, and there has been a demand for development of a material and a process that can produce such an article.
In addition, electronic apparatus such as household appliance and business equipment are flame-retarded in such a manner that wiring boards incorporated therein are neither fired nor burned. Therefore, a wiring board used in an electronic apparatus is required to have flame retardancy. As the specific degree of the flame retardancy, high flame retardancy that satisfies a severe standard value like V-0 prescribed in UL-94 (Flammability test of plastic materials for parts in devices and appliances) of UL standard (Underwriters' Laboratories, Inc. Standard) is required. Accordingly, there is a demand for articles such as MID, which can be highly flame-retarded as needed, in addition to the fact that they have such properties as described above.